He currently serves as the Senior Director for International Justice at Georgetown University, on a joint appointment between the Law Center and the Department of Government.

Ambassador Williamson heads the US Department of State-funded project at Georgetown that provides support to the Ukrainian government in its investigation and prosecution of crimes arising from the current conflict.

[7] Williamson served as the Special Assistant to the President George W. Bush and Senior Director for Relief, Stabilization, and Development at the National Security Council (NSC).

[8] From June 2010 to September 2011, Ambassador Williamson served as a Special Expert to the Secretary-General of the United Nations, focused primarily on the Extraordinary Chambers in the Courts of Cambodia.

The most prominent charges involved allegations that individuals associated with the Kosovo Liberation Army (KLA) murdered Serb prisoners for the purpose of harvesting and trafficking their bodily organs.

He stated that SITF's findings were largely consistent with those in the Marty Report, but that the investigation had not yet secured a level of evidence sufficient to charge those crimes.

[12] Upon his retirement from the U.S. Department of State in September 2014, Ambassador Williamson joined the faculty of the Sandra Day O'Connor College of Law at Arizona State University, where he served as a Distinguished Professor of Practice [8] and as Senior Director for International Rule of Law and Security at the university's McCain Institute for International Leadership, based in Washington, DC.